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As a result of enhancement and development in instrumental techniques in the past years, it has been reality to extend the detection limits of analytical methods for many kinds of heavy metals to be microgramme (μm) level and even below. However, public concern is increasing dramatically because of more and more health problems of drinking water have been found these years. For instance, in Africa, diarrhea remains a major health effect among children and even adults. Thus, as the primary step, analyzing the chemicals especially harmful chemicals is becoming more important for human health.
However, civilization and economy development is accompanied by an enhancement of environment contamination by many kinds of metals, among which cadmium is one of the most dangerous and toxic. In the field of industry, cadmium firstly exists as a minor component in most zinc ores and meanwhile is a by-product of zinc production. According to Kalicanin (2009), a small account of cadmium enter the environment from the natural weathering of minerals, forest fires, and volcanic emissions, but it is mainly released by human activities such as mining and smelting operations, fuel combustion, disposal of metal containing products, and application of phosphate fertilizer or sewage sludge. In addition, cadmium was utilized for a long time as pigment and for corrosion resistant planting on steel, and its compounds were used to stabilize plastic. Thus, cadmium is an essential industrial metal. Although the utilization of cadmium products has increased in these years at a rate of 5 to 10% each year, and the potential for further growth is still quite high; the use of cadmium should be generally controlled due to its high toxicity and carcinogenicity. In the environment water, there are three essential chemical forms of cadmium: solution composition (Cd2+), organic complexes (Cd-NOM) and inorganic complexes (CdCl+). The inorganic cation, hydrated (Cd2+¹’6H2O), exists to at the range of pH 7-8, therefore it is the major species present in freshwaters and seawaters. Chloro-complexes (CdCl+) and organic species (Cd-NOM, for example humic analogues) also exist dependent upon environmental conditions. Consequently, it is necessary to use cadmium carefully and have a complete and effective method to analysis. In this report, trace metal sample and analysis that focused on cadmium is mainly discussed; and it will be divided by several parts that contain the sample collection, sample extraction and feasible techniques for sample analysis and quality control procedures.
2. Sample collection
Firstly, samples for the analyses of cadmium are planned to collect from the treated effluent of Winmalee wastewater treatment plant which is located in the Blue Mountains catchment of Western Sydney. This effluent is discharged to the Nepean River, and the sample is one weekly interval from the next three months, Furthermore, other samples are collected from the Nepean River and the reservoir which is used to supply the drinking water for residents of Western Sydney.
In terms of sampling in Nepean River, grab sampling is selected primarily owing to the fact that our lab is to check randomly the water quality not to monitor all the time. Moreover, grab sampling is a common, easy sampling method and has low capital cost. Thus, the sampling method is decided to collect from surface, bottom and intermediate depths in Nepean River, and the sample frequency is chosen as twice or three times per week from May to September in 2010. In relation to sampling from the effluent of Winmalee wastewater treatment, although automatic sampling is suitable for sampling from stream flow, all metal speciation and bulk chemical parameter information will be lost, thus automatic sampling can only be applied to analyze total trace metal concentrations. For effluent sampling, the sample frequency is accepted as five times each day which lasts for ten days.
In terms of surface sampling, a plastic or a polypropylene bucket attached by a nylon rope is commonly used. However, there is a shortage of this technique which could collect heavy metal species from the surface film of the river or reservoir. Hence, according to Batley and Gardner (1977), it is recommended to use high density polyethylene bottles or jerry cans which can be immersed sufficiently below the surface. Regardless of any sampling devices, it is necessary to enable non-contaminating procedures for sample collection, thus all components of these sampling devices should be disinfected for two days with 5% hydrochloric acid (HCl) to remove surface contamination. In addition, it is also suggested to rinse thoroughly by distilled water rinse before sample collection.
Similar as the surface sampling, contamination from depth sampling device and procedures should completely be avoided. In relation to depth sampling from the Nepean River, the best materials of tracr metal samplers can be Polythene, polypropylene, polycarbonate, Teflon or Persex. Both for depth sampling and surface sampling, metal, neoprene rubber or other contaminating materials should not be used of samplers. In fact, according to Mitra (2003), polymer additives are also likely to leach out of sampling bottles, however, it can be ignored because no organic matter analysis is needed in this report.
3. Available techniques for sample extraction
Following the sample collection, it is not reasonable to directly analyze. The complete analytical process involves sampling, sample preservation (sample storage), sample preparation and analysis. Samples pretreatment is one of the most important facets of any analytical technique to prepare a sample appropriately prior to analysis. Poor sample preparation methodology can result in contamination or analyte loss. For trace metal analysis, filtration is the main process of pretreatment. Particulates (including algae and bacteria) must be firstly removed by filtration. After that, acidification and freezing are required in most cases. Finally, storage treatment should add preservative reagents to the sample, and then stored in an appropriate container under conditions. However, different with conventional sample preparation techniques, sample preparation for metal analysis by ion chromatography can include acid/alkali digestion or UV photolysis, solid phase extraction (SPE) to remove contaminant matrix ions using ion exchange or reversed phase C18 cartridges, dialysis, off- or on-line, metal preconcentration on small chelating columns, off- or on-line, addition of masking agents, for example EDTA, and/or dilution and selective sensitive analyte detection (Shaw and Haddad 2004). In this part, common techniques for sample extraction (sample pretreatment) will be introduced.
The experimental results of trace metal analysis are commonly detrimental influenced by suspended solids and soluble metal fraction of the sample. Moreover, in unfiltered sample, contact of the dissolved fraction with particles for extended periods of time is likely to result in changes in the distribution of chemical forms of heavy metals in solution. In addition, if there is a high bacterial concentration in sample, relatively high bacterial concentration associated with sedimentary martial will also induce both adsorptive loss and pH changes. Hence in this report, filtration is necessary since it is the main step of sample extraction. To be more specific, according to Mackey et al. (1997), an acid-cleaned 0.45μm Millipore filter in an acid-cleaned polycarbonate filter holder is put to use, while the suspended matter is retained by the filter. In addition to acid-clean devices, in order to avoid contamination during the filtration, it is beneficial to enable this process be performed in dust-free conditions, and it is preferable to be located in a clean room or at least in a laminar flow, clean hood (Mitchell 1973).
However, there is an obvious disadvantage of using filters of 0.45μm pore size: all phytoplankton and most bacteria which can be influenced on results are still retained. If continued filtration, it can contribute to clogging of pores with an efficiency reduction during the filtration process. After considering this, pressure filtration is more preferable to accept. Pressure filtration (Ultrafiltration) performs well in terms of efficiency because of its high filtration speed. According to Segar and Berberian (1975), pressure filtration is more preferable with freshwater samples, which has a high suspended sediment load. In this report, the samples are all from treated water and drinking water. Therefore, it is highly recommended to use pressure filtration with ultrafiltration membranes.
Although there is a filtration process during the sample pretreatment, it is necessary to pay more attention on oxidation. Due to the fact that most polyethylene containers only can inhibit but can not prevent gas diffusion, oxidation of anoxic samples may occur over time. Consequently, the freezing process should be added to freeze samples, which lead to reducing oxidation kinetics and minimizing the probability of oxidation.
In addition to sample pretreatment, sample storage is also essential to the whole process of analysis. Careful and reasonable sample storage means avoiding the sample losses. In terms of cadmium, using limited data available on storage losses of cadmium, it is suggested to use polythene containers with fridge to 4 Celsius degrees to achieve minimal losses. King et al. (1974) examined the losses of cadmium activity were negligible from Polythene, polypropylene and PVC bottles in the range of pH 3-10.
4. Current techniques for sampling analysis
There are many current analytical techniques used for measuring total trace metal concentrations (cadmium concentration) in water samples, which include potentiometric stripping analysis (PSA), atomic absorption spectrometry (AAS), inductively coupled plasma atomic emission spectroscopy (ICP-AES), and inductively coupled plasma mass spectrometry (ICP-MS). The success as well as the frequency of the mentioned above methods is different; the major difference is the limit of detection, and other differences include selectivity and reproducibility of the given technique, the rapidity and simplicity of the method as well as the cost of the device and its exploitation (Kalicanin 2009). In the modern time, most trace metal analyses conducted by inductively coupled plasma coupled with atomic emission spectroscopy or mass spectrometry. Therefore, in this part, focus is on the introduction of all available techniques including advantages and disadvantages description, while the suitable approach to enable accurate quantization of cadmium is introduced in the next part which can be compared with these techniques.
Potentiometric Stripping Analysis (PSA)
According to Kalicanin (2009), the Potentiometric stripping analysis (PSA) has relatively greater sensitivity (10−11mol/L), which is just after the neutron-activation analysis (10−21 mol/L). In addition to this, the cost of this technique is much lower than with the other mentioned above techniques, and the procedure for carrying out the analysis is relatively less time-consuming. A detection limit of the PSA of cadmium is 0.10μg/L. However, this technique is quite a new technique of analyzing the trace metal and it is not commonly applied. In addition to this, it can be limitation that the samples typically must be in solution.
Atomic Absorption Spectrometry
According to Michael and Bernhard (1999), atomic absorption spectroscopy is a technique for determining the concentration of a particular metal element in a sample. In more detail, the trace metals can be separated using absorption of light at a specific wavelength by excitation of an electron to higher orbital. In order to analyze atomic constituents of the samples, the sample has to be atomized firstly. After this, the sample should then be illuminated by light. Finally, a detector is used to measure the light transmitted. In addition, aim of reducing the effect of emission from the atomizer (e.g. the black body radiation) or the environment, a spectrometer is normally equipped between the atomizer and the detector. This technique can be used to analyze the concentration of over 70 different metals in a solution. However, the flame of AAS can only reach 2000-3000°C, the process atomization is not complete; similar with ICP system, matrix interferences can be generated to influence the accuracy of detection.
Inductively Coupled Plasma (ICP)
Inductively coupled plasma is the plasma which contains a sufficient concentration of ions and electrons to enable the gas electrically conductive. These plasmas used in analysis of spectrochemistry are essentially electrically neutral, with each positive charge on an ion balanced by a free electron. In these plasmas the positive ions are almost all singly-charged and the amount of negative ions are few, so there are nearly equal amounts of ions and electrons in each unit volume of plasma. Although there have been more energetic atomization techniques since the early 20th century, such as electric arc and spark sources, none of them have been routinely utilized until the plasma spectroscopy began to widely application in the 1970's.
There are many advantages of using inductively coupled plasma atomization techniques compared with using other atomization techniques, however, only the most important are listed below:
a. Many elements of trace metal can be determined simultaneously at the high temperatures of argon gas to create plasma.
b. An increased range of elements can be determined using this technique, and another importance is that phosphorus and sulfur can also be determined by this method in water laboratories.
c. In general, lower detection limits are achieved over flame spectroscopy, though graphite furnace has comparable and sometimes better detection limits.
d. When at the higher flame temperature compared with AES and AAS (2000-3000°C), atomization is more complete; in addition, and fewer chemical interference effects are encountered. Ionization interference effects are also small or non existent.
e. There are some other advantages in using ICP atomization over flame techniques. For instance, atomization occurs in a chemically inert atmosphere, enhancing the lifetime of the atomized analyte. Furthermore, the temperature cross-section is relatively uniform and so calibration curves tend to remain linear over several orders of magnitude.
All of these advantages can directly result in the high atomization temperatures achieved in the plasma, which can reach more than 7500 °C. This is accomplished by using argon gas over acetylene/nitrous oxide gas mixtures. At these higher temperatures most elements such as Cd+ are more than 90% ionized to their singly charged species, but second ionization potential is not achieved.
After the ICP system, there are two common detection systems which are called atomic emission spectroscopy (AES) and mass spectrometry (MS). Both of them will be introduced below.
Firstly, it is necessary to introduce the atomic emission spectroscopy (AES) detection system. It is a method of chemical analysis which uses the intensity of light emitted from a flame, plasma, arc, or spark at a particular wavelength to determine the quantity of an element in a sample. The wavelength of the atomic spectral line gives the identity of the element while the intensity of the emitted light is proportional to the number of atoms of the element.
Equipped with the ICP system, inductively coupled plasma atomic emission spectroscopy (ICP-AES) is a type of emission spectroscopy which uses the inductively coupled plasma to produce excited atoms and ions that emit electromagnetic radiation at wavelengths characteristic of a particular element. The intensity of this emission is indicative of the concentration of the element within the sample.
The advantages of ICP-AES are relatively excellent analytical detection limits and linear dynamic range, multi-element capability, low chemical interference and a stable and reproducible signal. However, there are some disadvantages which include spectral interferences (many emission lines), high cost and operating expense and the fact that samples typically must be in solution.
Inductively Coupled Plasma systems can also be equipped with a sensitive mass spectrometry (ICP-MS) detection system. According to lecture notes (2010), this equipment can separate ionized atoms by their mass-to-charge ratio which are then detected by an electron multiplier. This can lead to analytical instruments that have trace metal detection limits three to four-orders of magnitude lower than the other techniques. In ICP-MS, trace metals can be quantified by elemental abundance in general. In addition to its quite low detection limits, the advantages of ICP-MS include rapid and accurate detection, multi elemental capacity as well as a wide linear range (Sahan et al. 2007, Papaefthymiou et al. 2010).
However, there is a essential problem of interferences in trace metal quantification. The interferences include three possibilities. Firstly, as we know, mass spectrometry separates elements based on mass-to -charge ration. Some elements have the same mass which can not separate such as 64Ni and 64Zn. These isobaric interferences can be eliminated by analyzing other isotopes of the metal. Moreover, there is also a possible situation that polyatomic ions can form in the Ar plasma which is called polyatomic or molecular interferences. To avoid this, collision cells can be installed in ICP-MS instrumentation to overcome the interferences. Therefore, most recent ICP-MS instruments have a collision cell equipped before the quadropole, it will remove certain molecular interferences that occur in ICP-MS analyses.
A widely-used instrumentation is Agilent 7700 Series ICP-MS, its main parts include: sample introduction, plasma RF generator, electronic gas control, octopole ion guide, vacuum system, hyperbolic quardrupole, electron multiplier detector, octopole reaction system, off-axis ion lens as well as interface and cones.
5. Suitable approach for accurate quantization of cadmium analysis
Many of the analytical approaches can both measure total metal concentrations and trace metal speciation. For instance, ion chromatography can be applied to determine total metal concentration as well as provide speciation information. If only considering the accurate quantization of cadmium, ion chromatography seems more suitable approach to achieve.
According to Shaw and Haddad (2004), the term 'ion chromatography' encompasses the collection of analytical liquid chromatographic techniques used to separate and determine inorganic cations, anions and low molecular weight water-soluble organic acids and bases. There are mainly three approaches to the separation of metals using IC: ion exchange, ion interaction and chelation ion exchange. Timerbaev and Bonn (1993) stated that ion-exchange IC is the most obvious and thereby turn into the most common chromatographic method for the determination of metal ion.
Firstly, because cadmium is selected as the research chemical, it is necessary to know the Cd speciation in an environmental water sample. As mentioned previously, there are three important chemical forms of cadmium: solution composition (Cd2+), organic complexes (Cd-NOM) and inorganic complexes (CdCl+). The inorganic cation, hydrated (Cd2+¹’6H2O), exists to at the range of pH 7-8, therefore it is the major species present in freshwaters and seawaters. Chloro-complexes (CdCl+) and organic species (Cd-NOM, for example humic analogues) also occur dependent upon environmental conditions.
For the ion-exchange separation of divalent (Cd2+) and trivalent heavy and transition metal ions, both cation and mixed mode cation/anion based stationary phases are commonly used. Because of their effective charge density, the metals are frequently separated by secondary equilibria. Owing to the fact that the selectivity coefficients for metal ions of identical charge are very similar, making separations on cation exchangers difficult if not impossible; resultants can be strong complexing agents including dipicolinate and ethylenediamine, the other possible result is neutral or anionic complexes generated with weak complexing groups present in the eluent, including oxalate and tartrate(Shaw and Haddad 2004).
6. Data Analysis
1. Eluent Delivery
2. Sample injection
The flowchart of the ion chromatography
As the flowchart shown above, an ion chromatography system typically consists of a liquid eluent (mobile phase), a high pressure pump, a sample injector, a guard and separator column, a suppressor, a conductivity cell, and a data collection system. The whole process of ion chromatography is illustrated below. Before a sample introduced, the ion chromatography system firstly need to be calibrated using a series of standard solutions. By comparing the response obtained from the sample to that obtained from the standard calibration curve, analyte ions can be identified (by different retention time) and quantified. Then, the data analysis system produces a chromatogram, which typically refers to a computer running chromatography software. Finally, the chromatography software then analyzes and converts each peak in the chromatogram to the quantitative concentration of each element.
The main advantage of IC is its excellent detection limits. It is well known that the working range for the conductivity detection of cations and anions is typically in the mg/L range. However, most toxic trace metals including cadmium are found within the μg/L range in waters. In addition to this, water quality guideline values are also typically within the μg/L range. Hence, ion chromatography coupled to ICP-MS instruments obtains superior detection limits, which bypass the conductivity cell.
Using this methodology, the obtained data for practical sample analysis is much better compared with AAS and ICP-MS systems. According to Al-Shawi and Dahl (1999), they used the CS-5 column together with an oxalate eluent to determine Cd (II) and six other heavy metals in fertilizer solutions. The motherliqour solution was filtered and diluted prior to injection into the chromatograph, with detection limits of 1-30 ng (in 50 Al) (10 ng for Cd) reported using PAR detection. PAR is a popular reagent with a strong chromophore for transition and heavy metal detection.
Although the coupling of IC to ICP-MS has dramatically improved our ability to determine the speciation of many elements in water samples, there are also some limitations in its application. The most obvious limitation is that this methodology only performs well when analyzing stable species during chromatographic separation. For instance, many ionic metal complexes with inorganic and organic ligands are too labile to keep intact during the IC separation process.
6. Details of quality control procedures
As mentioned in the part 3, the complete analytical process involves sampling, sample preservation, sample preparation and analysis. The aim of quality control is to monitor, measure, and keep the systematic and random errors under control. There are three main aspects to achieve the quality control, which contains statistical control, matrix control and contamination control. In this part, contamination control will be emphasized to discuss.
As we know, many measurement processes are prone to contamination, which can occur at any point in the sampling, sample preparation, or analysis. Contamination has become the main limitation in trace metal analysis. Firstly, sampling devices can be the source of contamination. Thus mentioned in the part of sample collection, all components of these sampling devices should be soaked for two days with 5% hydrochloric acid (HCl) to remove surface contamination followed by distilled water rinse, as well as metal components should be Telfon-coated. Similarly in terms of sample extraction, an acid-cleaned 0.45μm Millipore filter in an acid-cleaned polycarbonate filter holder is put to use when operating filtration process of sampling extraction. In addition to sampling devices, contamination can also occur in the laboratory at the stage of analysis. In general, contamination can be reduced by avoiding manual sample by hand, as well as by reducing the number of discrete processing steps. However, these steps only have the limited ability to control contamination; in order to avoid and correct relatively constant, unavoidable contamination effectively, the analytical blanks should be used.
According to Mitra (2003), the term of blanks refer to samples that do not contain any analyte (sample). They are made to simulate the sample matrix as closely as possible. Depending on the process and the measurement objectives, different types of blanks are utilized. Blank samples from the laboratory and the field are ordinarily required to cover contamination from all possible processes. In this report, the blank which are essential from a sample preparation perspective is focused on and will be accepted in our lab. Of all blanks, system or instrument blank is a measure of system contamination and is to establish the baseline of an instrument when no sample present. This blank will determine the background signal with no sample present; the current level is confirmed as the zero setting when the signal is constant. This type of blank is suitable both for analytical instruments and sample preparation devices.